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Identification of tests which are candidates for automation is a fine art and a dark science. The tests must be ones which are repeatedly required in order to justify the time and expenditure on development of manual tests, and they must have clear inputs and outputs. Unit tests are a good example, but also, for instance, functional tests on an e-commerce website that relate to calculation of price, or are conditional on the display of a predictable, clearly-identified page element. Given these parameters, the decision of whether to include tests for automation is a budgetary, time-related one.

Conversely, look and feel tests, most changes to the UI and cross-platform tests are not usually good candidates for automation.

All the scenarios which comes under regression testing will be automated and the other will be done by manual testing.

All those test cases which are related to UI/LookandFeel - so that there is no manual time used.
Those which are static but important to the flow - For ex preconditions
All those which requires data driven tests - For ex. Login with different category of ACLs
Regression suite - to avoid redundancy of tasks
Other cases which are too frequently /repeatedly checked